1. Field of the Invention
The present invention relates to a method and system for automatically controlling a fluid jet, and, in particular, to methods and systems for automatically controlling lead, taper, and other orientation and process parameters of a high pressure waterjet using predictive models.
2. Background
High-pressure fluid jets, including high-pressure abrasive waterjets, are used to cut a wide variety of materials in many different industries. Abrasive waterjets have proven to be especially useful in cutting difficult, thick, or aggregate materials, such as thick metal, glass, or ceramic materials. Systems for generating high-pressure abrasive waterjets are currently available, for example the Paser 3 system manufactured by Flow International Corporation, the assignee of the present invention. An abrasive jet cutting system of this type is shown and described in Flow""s U.S. Pat. No. 5,643,058, which is incorporated herein by reference. The terms xe2x80x9chigh-pressure fluid jetxe2x80x9d and xe2x80x9cjetxe2x80x9d used throughout should be understood to incorporate all types of high-pressure fluid jets, including but not limited to, high-pressure waterjets and high-pressure abrasive waterjets. In such systems, high-pressure fluid, typically water, flows through an orifice in a cutting head to form a high-pressure jet, into which abrasive particles are combined as the jet flows through a mixing tube. The high-pressure abrasive waterjet is discharged from the mixing tube and directed toward a workpiece to cut the workpiece along a designated path.
Various systems are currently available to move a high-pressure fluid jet along a designated path. Such systems are commonly referred to as three-axis and five-axis machines. Conventional three-axis machines mount the cutting head assembly in such a way that it can move along an x-y plane and perpendicular along a z-axis, namely toward and away from the workpiece. In this manner, the high-pressure fluid jet generated by the cutting head assembly is moved along the designated path in an x-y plane, and is raised and lowered relative to the workpiece, as may be desired. Conventional five-axis machines work in a similar manner but provide for movement about two additional rotary axes, typically about one horizontal axis and one vertical axis so as to achieve in combination with the other axes, degrees of tilt and swivel.
Manipulating a jet about five axes may be useful for a variety of reasons, for example, to cut a three-dimensional shape. Such manipulation may also be desired to correct for cutting characteristics of the jet or for the characteristics of the cutting result. More particularly, as understood by one of ordinary skill in the art, a cut produced by a jet, such as an abrasive waterjet, has characteristics that differ from cuts produced by more traditional machining processes. Two of the cut characteristics that may result from use of a high-pressure fluid jet are referred to as xe2x80x9ctaperxe2x80x9d and xe2x80x9ctrailback.xe2x80x9d FIG. 1 is an example illustration of taper. Taper refers to the angle of a plane of the cut wall relative to a vertical plane. Taper typically results in a target piece that has different dimensions on the top surface (where the jet enters the workpiece) than on the bottom surface (where the jet exits the workpiece). FIG. 2 is an example illustration of trailback. Trailback, also referred to as drag, identifies the phenomena that the high-pressure fluid jet exits the workpiece at a point behind the point of entry of the jet into the workpiece, relative to the direction of travel. These two cut characteristics, namely taper and trailback, may or may not be acceptable, given the desired end product. Taper and trailback varies depending upon the speed of the cut; thus, one known way to control excessive taper and/or trailback is to slow down the cutting speed of the system. In situations where it is desirable to minimize or eliminate taper and trailback, conventional five-axis systems have been used, primarily through manual trial and error, to apply taper and lead angle corrections to the jet as it moves along the cutting path.
In brief summary, methods and systems of the present invention provide for the automatic control of orientation parameters of a fluid jet to achieve greater control over the contour of the cut produced and the resultant piece. These methods and systems can be employed with different types of jet apparatus, such as those that control a cutting head using motion around a different number of axes. Example embodiments provide a Dynamic Waterjet Control System (xe2x80x9cDWCSxe2x80x9d) to dynamically control the orientation of a jet relative to the material being cut as a function of speed and/or other process parameters. Orientation parameters include, for example, the x-y position of the jet along the cutting path, as well as three dimensional orientation parameters of the jet, such as the standoff compensation values and the taper and lead angles of the cutting head. In one embodiment, the DWCS uses a set of predictive models to automatically determine appropriate orientation parameters for an arbitrary geometry as functions of speed. In this manner, these models dynamically match, for each geometric entity, the speed of the cutting head to appropriate lead and taper angles under differing process conditions of the cutting head. For example, when a corner is being cut, typically the cutting head is slowed. In some cases, using the automated lead and taper angle determination techniques of the present invention, the deceleration may be lessened, while the cutting head achieves a more accurate cut.
In one embodiment, the DWCS comprises a user interface; which may be implemented as a graphical user interface (a xe2x80x9cGUIxe2x80x9d); a motion program generator; one or more replaceable models; and a communications interface to a controller of the cutting head. The DWCS optionally provides CAD capabilities for designing the target piece or receives CAD input by other means. In some embodiments, the DWCS resides in a separate computer workstation; while in other embodiments the DWCS resides on the controller, or a computer associated therewith.
The motion program generator dynamically generates a motion program for a controller of a jet apparatus. The generated motion instructions are dependent upon the requirements of the controller and/or the jet apparatus and, thus, the motion program generator can be tailored to generate differing types of control instructions for each type of controller.
The motion program generator automatically determines the lead and taper angle adjustments for each geometric entity as a function of the determined speed for that entity. In one embodiment, the lead and taper angle adjustments are functions of other process parameters, such as mixing tube length or orifice diameter. In another embodiment, a speed and acceleration model is used by the DWCS to determine the speed for an entity prior to determining the lead and taper angle adjustments. In some embodiments the lead and taper angle adjustments are determined at the same time as speed adjustments.
The model used by techniques of the present invention models the contour of the cut that can be achieved under varying conditions, as specified by different process parameter values. Any technique for providing values for lead and taper for an arbitrary geometry can be used to implement the lead and taper model. In some embodiments, the lead and taper model comprises sets of polynomial equations. In other embodiments, the lead and taper model comprises a look-up table of discrete values that models lead and taper angles for a set of geometries. In some embodiments, the lead and taper model models lead and taper angles as functions of speed and material thickness. In addition, one embodiment includes an angle of a tangent to the path at the current endpoint to support the determination of smoother transitions around entities such as corners or other intersections.
In yet another embodiment, the lead and taper angles can be manually overridden by an operator for a portion of or the entire cutting path. Additionally, the automated lead and taper angle adjustment can operate in conjunction with manual override of some parameters, but not others.
In some embodiments, some or all of the process of automatically determining one or more of the orientation parameters and controlling the cutting head accordingly are performed by the controller of the jet apparatus or software/hardware/firmware directly connected to the controller.